On Fri, Jul 20, 2018 at 12:32:10PM -0700, Andy Lutomirski wrote: > I'm just reading your changelog, and you said the PMDs are no longer > shared between the page tables. So this presumably means that > vmalloc_fault() no longer actually works correctly on PTI systems. I > didn't read the code to figure out *why* it doesn't work, but throwing > random vmalloc_sync_all() calls around is wrong. Hmm, so the whole point of vmalloc_fault() fault is to sync changes from swapper_pg_dir to process page-tables when the relevant parts of the kernel page-table are not shared, no? That is also the reason we don't see this on 64 bit, because there these parts *are* shared. So with that reasoning vmalloc_fault() works as designed, except that a warning is issued when it's happens in the NMI path. That warning comes from ebc8827f75954 x86: Barf when vmalloc and kmemcheck faults happen in NMI which went into 2.6.37 and was added because the NMI handler were not nesting-safe back then. Reason probably was that the handler on 64 bit has to use an IST stack and a nested NMI would overwrite the stack of the upper handler. We don't have this problem on 32 bit as a nested NMI will not do another stack-switch there. I am not sure about 64 bit, but there is a lot of assembly magic to make NMIs nesting-safe, so I guess the problem should be gone there too. Regards, Joerg
